Ultrastructural and immunohistochemical studies were made to analyze the evolution of fibrotic and vascular changes in pulmonary silicosis in rats. Early lesions of intraluminal exudation of inflammatory cells and fibrin became granulomatous and then were lined by proliferating alveolar epithelial cells. Two patterns of endothelial cell migration were demonstrated: the first pattern was characterized by endothelial cells that extended their cytoplasm over preexisting, denuded basement membranes and replaced necrotic cells in alveolar capillaries; the second pattern consisted of bud-like sprouts which developed only in peribronchiolar connective tissue. Our observations indicate that peribronchiolar vessels are sources for renewal of alveolar capillary endothelium as well as for neovascularization.